Systems and methods for oxidation of hydrocarbon gases

ABSTRACT

The present disclosure relates to systems and methods wherein a dilute hydrocarbon stream can be oxidized to impart added energy to a power production system. The oxidation can be carried out without substantial combustion of the hydrocarbons. In this manner, dilute hydrocarbon streams that would otherwise be required to undergo costly separation processes can be efficiently utilized for improving the power production system and method. Such systems and methods particularly can utilize dilute hydrocarbon stream including a significant amount of carbon dioxide, such as may be produced in hydrocarbon recovery process, such as enhanced oil recovery or conventional hydrocarbon recovery processes.

FIELD OF THE DISCLOSURE

The present disclosure relates to systems and methods whereby variousgas streams may be oxidized for energy production. A gas stream moreparticularly may include hydrocarbons and one or more diluents. Energyproduced via oxidation may be imparted, for example, to a powerproduction system or method.

BACKGROUND

Many industrial processes result in gaseous streams that include acontent of hydrocarbon materials that are combustible. In manyinstances, such streams may include hydrocarbons as well as one or morefurther material that may be considered to contaminate or otherwisedilute the hydrocarbons and thus limit their usefulness. Carbon dioxideis an example of a further material that is frequently found comingledwith hydrocarbon gases, particularly in various aspects of the petroleumindustry. For example, raw natural gas produced from geologic formationsoften includes a significant content of carbon dioxide. Conversely,carbon dioxide withdrawn from geologic formations often includes asignificant content of hydrocarbon gases. Still further, productiongases that are recovered in enhanced oil recovery (EOR) methods oftencomprise a mixture of hydrocarbon gases and carbon dioxide.

Combination gas streams, such as the examples noted above, typicallyrequire specific processing in order to separate the components—i.e., toprovide a substantially pure hydrocarbon stream that may be suitable forcombustion and/or to provide a substantially pure carbon dioxide streamthat may be suitable for use in EOR, or sequestration, or for other enduses. It thus would be useful to have further options for utilizinghydrocarbon-containing streams.

SUMMARY OF THE DISCLOSURE

In one or more embodiments, the present disclosure can provide systemsand methods useful for power production. In particular, the systems andmethods can be configured for processing of a dilute hydrocarbon streamsuch that the hydrocarbons in the stream are oxidized withoutsubstantial combustion and thusly impart added energy to the powerproduction cycle.

In one or more embodiments, the present disclosure can provide a methodfor power production comprising:

combusting a carbonaceous fuel with oxygen in a combustor in thepresence of a recycle CO₂ stream to form a combustion product streamcomprising CO₂;

expanding the combustion product stream in a turbine to produce powerand form a turbine exhaust stream;

cooling the turbine exhaust stream in a recuperator heat exchanger;

removing any water present from the cooled turbine exhaust stream toform the recycle CO₂ stream;

compressing at least a portion of the recycle CO₂ stream;

optionally diverting a portion of the recycle CO₂ stream and combiningthe diverted portion with the oxygen prior to said combusting;

passing the compressed recycle CO₂ stream back through the recuperatorheat exchanger such that the compressed recycle CO₂ stream is heatedwith heat withdrawn from the turbine exhaust stream;

inputting a dilute hydrocarbon stream under conditions wherein thehydrocarbons in the dilute hydrocarbon stream are oxidized withoutsubstantial combustion; and

directing the heated, compressed recycle CO₂ stream to the combustor.

In one or more further embodiments, the method for power production canbe further defined in relation to one or more of the followingstatements, which can be combined in any number and order.

The dilute hydrocarbon stream can be input such that the hydrocarbonsare oxidized within the recuperator heat exchanger or a further heatexchanger configured for heat exchange with one or both of the recycleCO₂ stream and the oxygen.

The dilute hydrocarbon stream can be combined with the compressedrecycle CO₂ stream before said passing step.

The dilute hydrocarbon stream can be combined with the compressedrecycle CO₂ stream in the recuperator heat exchanger.

The dilute hydrocarbon stream can be combined with the compressedrecycle CO₂ stream in a further heat exchanger.

A portion of the recycle CO₂ stream can be diverted and combined withthe oxygen to form a diluted oxygen stream, and the dilute hydrocarbonstream can be combined with the diluted oxygen stream.

The diluted oxygen stream combined with the dilute hydrocarbon streamcan be passed through the recuperator heat exchanger or a further heatexchanger wherein the hydrocarbons in the dilute hydrocarbon stream areoxidized.

The dilute hydrocarbon gas is a product of an enhanced oil recoveryprocess.

In one or more embodiments, the present disclosure can provide a methodfor power production comprising:

-   -   carrying out a closed or semi-closed Brayton cycle wherein:        -   CO₂ is used as a working fluid;        -   a carbonaceous fuel is used as a first fuel source and is            combusted to heat the working fluid; and        -   a recuperator heat exchanger is used to re-capture heat of            combustion; and    -   adding a dilute hydrocarbon stream to the closed or semi-closed        Brayton cycle as a second fuel source, wherein hydrocarbons in        the dilute hydrocarbon stream are oxidized without substantial        combustion to provide additional heat.

In one or more embodiments, the present disclosure can provide a methodfor processing of a waste stream comprising:

-   -   providing a waste stream comprising one or more hydrocarbons and        one or more diluents;    -   inputting the waste stream into a closed or semi-closed Brayton        cycle such that the hydrocarbons in the waste stream are        oxidized without substantial combustion.

In one or more embodiments, a method for power production can comprisecarrying out a closed or semi-closed power production cycle wherein: CO₂is circulated as a working fluid that is repeatedly compressed andexpanded for power production; a first fuel source is combusted in acombustor to heat the working fluid after the working fluid iscompressed and before the working fluid is expanded for powerproduction; and a recuperator heat exchanger is used to re-capture heatof combustion for heating of the working fluid. The method further cancomprise heating the working fluid with heat that is formed outside ofthe combustor using a second fuel source, said heating being in additionto the re-captured heat of combustion, and said second fuel source beinga dilute hydrocarbon stream that is oxidized without substantialcombustion to provide the heat that is formed outside of the combustor.

In one or more further embodiments, the method for power production canbe further defined in relation to one or more of the followingstatements, which can be combined in any number and order.

The concentration of hydrocarbons in the dilute hydrocarbon stream canbe below the lower explosive limit (LEL) of the hydrocarbons.

Hydrocarbons in the dilute hydrocarbon stream can be catalyticallyoxidized.

The method particularly can comprise the following steps: the first fuelis combusted with oxygen in the combustor in the presence of the CO₂working fluid to form an exhaust stream; the exhaust stream from thecombustor is expanded in a turbine to produce power and form a turbineexhaust stream; the turbine exhaust stream is cooled in the recuperatorheat exchanger; the turbine exhaust stream exiting the recuperator heatexchanger is purified to remove at least water from the working fluid;at least a portion of the working fluid is compressed in a compressor;at least a portion of the compressed working fluid is passed backthrough the recuperator heat exchanger such that the compressed workingfluid is heated with heat withdrawn from the turbine exhaust stream; andthe heated, compressed working fluid is recirculated to the combustor.

The dilute hydrocarbon stream can be added to the working fluid afterthe working fluid is compressed in the compressor and before the workingfluid is passed back through the recuperator heat exchanger.

The hydrocarbons in the dilute hydrocarbon stream can be oxidized withinthe recuperator heat exchanger.

The hydrocarbons in the dilute hydrocarbon stream can be oxidized in afurther heat exchanger configured for heat exchange with one or both ofthe working fluid and an oxygen stream providing the oxygen to thecombustor.

The dilute hydrocarbon stream can be combined with the compressedworking fluid in the recuperator heat exchanger.

A portion of the compressed working fluid can be combined with oxygen toform a diluted oxygen stream, and wherein the dilute hydrocarbon streamis combined with the diluted oxygen stream.

The diluted oxygen stream combined with the dilute hydrocarbon streamcan be passed through the recuperator heat exchanger wherein thehydrocarbons in the dilute hydrocarbon stream are oxidized.

The diluted oxygen stream combined with the dilute hydrocarbon streamcan be passed through a further heat exchanger wherein the hydrocarbonsin the dilute hydrocarbon stream are oxidized.

The dilute hydrocarbon stream can be input to an oxidation reactor.

A reaction stream exiting the oxidation reactor can be input to therecuperator heat exchanger.

A reaction stream exiting the oxidation reactor is input to a furtherturbine for power production.

A portion of the turbine exhaust stream can be input to the oxidationreactor so as to be included in the reaction stream that is input to thefurther turbine.

The dilute hydrocarbon stream can be a product of an enhanced oilrecovery process.

In one or more embodiments, the present disclosure can provide a systemfor power production comprising:

a power production unit configured for carrying out a closed orsemi-closed Brayton cycle, said unit including a combustor configuredfor combustion of a carbonaceous fuel in the presence of a recycle CO₂stream; and

one or more inputs configured for input of a dilute hydrocarbon streamto a component of the unit other than the combustor.

In some embodiments, a power production system can comprise: a powerproduction unit configured for carrying out a closed or semi-closedpower production cycle, said power production unit including: acombustor configured for combustion of a first fuel in the presence of acompressed CO₂ working fluid; a turbine configured for expanding thecompressed CO₂ working fluid to provide an expanded CO₂ working fluid; acompressor configured for compressing the expanded CO₂ working fluid toprovide the compressed CO₂ working fluid; a recuperator heat exchangerconfigured for transferring heat from the expanded CO₂ working fluidleaving the turbine to the compressed CO₂ working fluid leaving thecompressor; and one or more inputs configured for input of a dilutehydrocarbon stream to a component of the power production unit otherthan the combustor.

In one or more further embodiments, the power production system can befurther defined in relation to one or more of the following statements,which can be combined in any number and order.

The input can be configured for input of the dilute hydrocarbon streaminto the recuperator heat exchanger.

The power production system can further comprise a second heatexchanger, and the input can be configured for input of the dilutehydrocarbon stream into the second heat exchanger.

The input can be configured for input of the dilute hydrocarbon streaminto a line comprising the CO₂ working fluid.

The input can be configured for input of the dilute hydrocarbon streaminto the line downstream of the recuperator heat exchanger and upstreamof the compressor.

The power production system further can comprise an oxidation reactor,and the input can be configured for input of the dilute hydrocarbonstream into the oxidation reactor.

The oxidation reactor can be a catalytic oxidation reactor.

The oxidation reactor can be configured for output of a reaction streamthat is input to the recuperator heat exchanger.

The oxidation reactor can be configured for receiving a portion of theexpanded CO₂ working fluid upstream of the recuperator heat exchanger.

The power production system further can comprise a second turbineconfigured for receiving a reaction stream from the oxidation reactor.

BRIEF SUMMARY OF THE FIGURES

Having thus described the disclosure in the foregoing general terms,reference will now be made to accompanying drawings, which are notnecessarily drawn to scale, and wherein:

FIG. 1 is a flow diagram for a power production plant according toembodiments of the present disclosure wherein a dilute hydrocarbonstream can be input to various elements of the power production plant;

FIG. 2 is a flow diagram for a power production plant according toembodiments of the present disclosure wherein a dilute hydrocarbonstream is input to a recycled working fluid stream;

FIG. 3 is a flow diagram for a power production plant according toembodiments of the present disclosure wherein a dilute hydrocarbonstream is input to a supplemental heat exchanger;

FIG. 4 is a flow diagram for a power production plant according toembodiments of the present disclosure wherein a dilute hydrocarbonstream is combined with a diluted oxidant stream;

FIG. 5 is a flow diagram for a power production plant according toembodiments of the present disclosure wherein a dilute hydrocarbonstream is input to a catalytic reactor;

FIG. 6 is a flow diagram for a power production plant according toembodiments of the present disclosure wherein a dilute hydrocarbonstream is input to a turbine exhaust stream downstream from a turbineand upstream from a heat exchanger; and

FIG. 7 is a flow diagram for a power production plant according toembodiments of the present disclosure wherein a dilute hydrocarbonstream is input to a catalytic reactor with a turbine exhaust stream andthen expanded through a secondary turbine.

DETAILED DESCRIPTION

The present invention now will be described more fully hereinafter. Thisinvention may, however, be embodied in many different forms and shouldnot be construed as limited to the embodiments set forth herein; rather,these embodiments are provided so that this disclosure will be thoroughand complete, and will fully convey the scope of the invention to thoseskilled in the art. As used in this specification and the claims, thesingular forms “a,” “an,” and “the” include plural referents unless thecontext clearly dictates otherwise.

In one or more embodiments, the present disclosure provides systems andmethods for power production wherein a dilute hydrocarbon stream isoxidized without substantial combustion to add energy for powerproduction. The systems and methods allow for low cost, efficientutilization of streams that would otherwise be required to undergoexpensive and time-consuming separation to provide useful materials(e.g., a purified stream of the hydrocarbons and/or a purified stream ofone or more diluents).

A dilute hydrocarbon stream as used herein is understood to mean astream that comprises greater than trace amounts of one or morehydrocarbons and at least one diluent. The dilute hydrocarbon stream cancomprise a concentration of hydrocarbons that is suitable to provide thedesired level of heating via oxidation of the hydrocarbons. Theconcentration of the hydrocarbons in the dilute hydrocarbon stream islimited only in that the dilute hydrocarbon stream compriseshydrocarbons in an amount that is below the lower explosive limit (LEL).In particular, the dilute hydrocarbon concentration in the stream can bebelow the LEL after the dilute hydrocarbon is mixed with a recycle CO₂stream as further described herein.

A hydrocarbon present in the dilute hydrocarbon stream preferably is ina gaseous state. Non-limiting examples of hydrocarbons that may bepresent include C₁ to C₁₀ compounds. Preferably, the dilute hydrocarbonstream comprises C₁ to C₄ compounds; however, C₅ to C₁₀ compounds may bepresent, particularly when the dilute hydrocarbon stream will be subjectto pressurization. In specific embodiments, the dilute hydrocarbonstream comprises at least methane. In some embodiments, the dilutehydrocarbon stream comprises natural gas.

A diluent present in the dilute hydrocarbon stream can be any materialthat serves to dilute the hydrocarbon concentration to be within therange noted above. In specific embodiments, the diluent can compriseCO₂. Other non-limiting examples of diluents that may be present includenitrogen, water, H₂S, and oxygen. In some embodiments, the diluent cancomprise predominately CO₂ (i.e., greater than 50% by volume of thediluent being CO₂), and the diluent specifically can comprise about 60%by volume or greater, about 75% by volume or greater, about 80% byvolume or greater, about 90% by volume or greater, about 95% by volumeor greater, about 98% by volume or greater, about 99% by volume orgreater, about 99.5% by volume or greater, or about 99.8% by volume orgreater CO₂. For example, the diluent can comprise about 60% by volumeto about 99.9% by volume, about 75% by volume to about 99.8% by volume,or about 80% by volume to about 99.5% by volume CO₂.

The dilute hydrocarbon stream can come from any source, includingindustrial waste, reaction products, hydrocarbon production streams(e.g., from a natural gas well or oil well) and the like. If desired, ahydrocarbon stream from such source can be specifically diluted throughaddition of CO₂ (or other diluent) to the hydrocarbon stream. Forexample, a waste stream can comprise hydrocarbons in a concentrationabove the LEL, and such stream may be used according to the presentdisclosure through addition of further diluent. Further to the above,CO₂ withdrawn from natural formations often include a content of naturalgas or other mixture of gaseous hydrocarbons. In some embodiments, adilute hydrocarbon stream can arise from enhanced oil recovery (EOR),such as in methods described in U.S. Pat. No. 8,869,889 to Palmer etal., the disclosure of which is incorporated herein by reference. EORmethods typically result in a production stream comprising a mixture ofmaterials that must be separated to provide useable streams ofsubstantially pure materials. When CO₂ is used in EOR, the producedmaterials specifically must be treated to separate CO₂ from thehydrocarbon products. In the aforementioned patent to Palmer et al., amixture of CO₂ and hydrocarbons may be used as part of the fuel sourcein an associated power production method. In such methods, the combinedCO₂ and hydrocarbon mixture is directed to a combustor where it iscombusted, typically along with a stream of substantially purehydrocarbon fuel. Such method requires a purpose-built combustor capableof combusting lower BTU-content fuels, is limited to using only streamswith a specified hydrocarbon centration (in order to maintain flamestability in the combustor), and is limited in the total flowrate ofCO₂-rich hydrocarbons that can be processed in a single plant. Further,because the relative concentrations of the components of such stream ofCO₂ and gaseous hydrocarbons from EOR can undergo significantfluctuations, such methods are hindered in that it is difficult toachieve a substantially constant flame temperature in the combustor.

Currently, significant energy and expense is required to separatehydrocarbons from CO₂. In the case of raw natural gas production, thedilute hydrocarbons produced from the field are typically dried,distilled to remove longer chain hydrocarbons (natural gas liquids, orNGLs), sweetened via removal of H₂S and other impurities, and sentthrough an absorber tower to scrub out the CO₂. The cleaned natural gasis then sent into the pipeline for downstream consumption, such as bypower production, and the clean CO₂ is vented, sequestered, and/orutilized (e.g., for further EOR). When CO₂ is used for EOR, the portionof the injected CO₂ that is produced along with the produced oil oftencontains a small amount of gaseous hydrocarbons that must be separatedin order to enable reinjection of the CO₂ into the formation. ThisCO₂-rich hydrocarbon gas must be separated from produced oil andsimilarly dried and distilled to remove any NGLs. This gas must then berecompressed for reinjection into the field. These processes require alarge amount of energy and consumables, leading to high capital expensesand operating expenses for the process.

The systems and methods of the present disclosure allow for low cost,efficient use of dilute hydrocarbon streams to add energy to existingpower production systems and methods. For example, the dilutehydrocarbon streams can be input to a system and method wherein acarbonaceous fuel is combusted to produce heat to a stream that may ormay not be pressurized above ambient pressure. The dilute hydrocarbonstream likewise can be applied to one or more systems wherein a workingfluid is circulated for being repeatedly heated and cooled and/or forbeing repeatedly pressurized and expanded. Such working fluid cancomprise one or more of H₂O, CO₂, and N₂, for example.

The systems and methods of the present disclosure can overcome problemsin the field by extracting the heating value of the entrainedhydrocarbons of a dilute hydrocarbon stream without combustion. Instead,the inherent conditions of the high temperature power production systemsand methods can be utilized to facilitate thermal oxidation of thesehydrocarbons in the dilute hydrocarbon stream. For example, oxidationcan occur in a heat exchanger. This allows the existing power cycles toutilize these dilute hydrocarbon streams with minimal modification ofthe process, to utilize significantly higher flow rates of thesestreams, and to simplify the overall cycle by eliminating certainequipment and the need for external sources of heat.

Utilization of CO₂ (particularly in supercritical form) as a workingfluid in power production has been shown to be a highly efficient methodfor power production. See, for example, U.S. Pat. No. 8,596,075 to Allamet al., the disclosure being incorporated herein by reference, whichdescribes the use of a directly heated CO₂ working fluid in arecuperated oxy-fuel Brayton cycle power generation system withvirtually zero emission of any streams to the atmosphere. It haspreviously been proposed that CO₂ may be utilized as a working fluid ina closed cycle wherein the CO₂ is repeatedly compressed and expanded forpower production with intermediate heating using an indirect heatingsource and one or more heat exchangers. See, for example, U.S. Pat. No.8,783,034 to Held, the disclosure of which is incorporated herein byreference. Thus, in some embodiments, the dilute hydrocarbon stream canbe used an input to a closed or semi-closed Brayton cycle to increasethe efficiency of power production via such cycle.

Further examples of power production systems and methods wherein adilute hydrocarbon stream as described herein can be used are disclosedin U.S. Pat. No. 9,068,743 to Palmer et al., U.S. Pat. No. 9,062,608 toAllam et al., U.S. Pat. No. 8,986,002 to Palmer et al., U.S. Pat. No.8,959,887 to Allam et al., U.S. Pat. No. 8,869,889 to Palmer et al., andU.S. Pat. No. 8,776,532 to Allam et al., the disclosures of which areincorporated herein by reference. As a non-limiting example, a powerproduction system with which a dilute hydrocarbon stream may be used canbe configured for combusting a fuel with O₂ in the presence of a CO₂circulating fluid in a combustor, preferably wherein the CO₂ isintroduced at a pressure of at least about 12 MPa (e.g., about 12 MPa toabout 60 MPa) and a temperature of at least about 400° C. (e.g., about400° C. to about 1,200° C.), to provide a combustion product streamcomprising CO₂, preferably wherein the combustion product stream has atemperature of at least about 800° C. (e.g., about 1,500° C.). Suchpower production system further can be characterized by one or more ofthe following:

The combustion product stream can be expanded across a turbine with adischarge pressure of about 1 MPa or greater (e.g., about 1 MPa to about7.5 MPa) to generate power and provide a turbine discharge steamcomprising CO₂.

The turbine discharge stream can be passed through a recuperator heatexchanger unit to provide a cooled discharge stream.

The cooled turbine discharge stream can be processed to remove one ormore secondary components other than CO₂ (particularly any water presentand/or SO_(x) and/or NO_(x)) to provide a purified discharge stream,which particularly may be a recycle CO₂ stream.

The recycle CO₂ stream can be compressed, particularly to a pressurewherein the CO₂ is supercritical.

The supercritical CO₂ can be cooled to increase the density (preferablyto at least about 200 kg/m³) of the recycle CO₂ stream.

The high density recycle CO₂ stream can be pumped to a pressure suitablefor input to the combustor (e.g., as noted above).

The pressurized recycle CO₂ stream can be heated by passing through therecuperator heat exchanger unit using heat recuperated from the turbinedischarge stream.

All or a portion of the pressurized recycle CO₂ stream can be furtherheated with heat that is not withdrawn from the turbine discharge stream(preferably wherein the further heating is provided one or more of priorto, during, or after passing through the recuperator heat exchanger)prior to recycling into the combustor.

The heated pressurized recycle CO₂ stream can be passed into thecombustor.

In one or more embodiments, a power production system suitable for inputof a dilute hydrocarbon stream as described herein can be configured forheating via methods other than through combustion of a carbonaceous fuel(or in addition to combustion of a carbonaceous fuel). As onenon-limiting example, solar power can be used to supplement or replacethe heat input derived from the combustion of a carbonaceous fuel in acombustor. Other heating means likewise can be used. In someembodiments, any form of heat input into a CO₂ recycle stream at atemperature of 400° C. or less can be used. For example, condensingsteam, gas turbine exhaust, adiabatically compressed gas streams, and/orother hot fluid streams which can be above 400° C. may be utilized.

In one or more embodiments, a power production plant may include somecombination of the elements shown in FIG. 1 (although it is understoodthat further elements may also be included). As seen therein, a powerproduction plant 10 (or power production unit) can include a combustor100 configured to receive fuel from a fuel supply 50 (e.g., acarbonaceous fuel) and oxidant from an oxidant supply 60 (e.g., an airseparation unit or plant (ASU) producing substantially pure oxygen). Aplurality of fuel supply lines (52, 54) are illustrated; however, only asingle fuel supply line may be used, or more than two fuel supply linesmay be used. Likewise, while only a single oxidant line 62 isillustrated, a plurality of oxidant lines may be used. The fuel iscombusted in the combustor with the oxidant in the presence of a recycleCO₂ stream. The combustion product stream in line 102 is expanded acrossa turbine 110 to produce power with a combined generator 115. Althoughthe combustor 100 and turbine 110 are illustrated as separate elements,it is understood that, in some embodiments, a turbine may be configuredso as to be inclusive of the combustor. In other words, a single turbineunit may include a combustion section and an expansion section.Accordingly, discussion herein of passage of streams into a combustormay also be read as passage of streams into a turbine that is configuredfor combustion as well as expansion.

Turbine exhaust in line 112 is cooled in a heat exchanger 120, and water(in line 132) is separated in separator 130 to produce a substantiallypure recycle CO₂ stream in line 135. If desired, part of the stream ofsubstantially pure CO₂ may be withdrawn from the plant and/or divertedto other parts of the plant (e.g., for cooling the turbine). The recycleCO₂ stream is compressed in a multi-stage compressor. As illustrated,the multi-stage compressor includes a first stage 140, a second stage160, and an intercooler 150. Optionally, one or more further compressorsor pumps may be added. The compressed recycle CO₂ stream in line 165 ispassed back through the heat exchanger 120 to the combustor 100. Asillustrated (and as further discussed below), a dilute hydrocarbonstream 170 can be introduced into the power production cycle. The stream170 is shown generally as one or more inputs configured for input of thedilute hydrocarbon stream to a component of the power production unit10. This is illustrated by the solid arrow on the right margin ofFIG. 1. The dilute hydrocarbon stream 170 specifically may be excludedfrom being input to the combustor 100.

Within the power production cycle as discussed above, the recycle streamin one or both of line 135 and line 165 (consisting of predominantlyclean CO₂) can be divided into an export CO₂ fraction, a diluting CO₂fraction, and a recycle CO₂ stream. The ratio of the CO₂ divided intothe diluting CO₂ fraction is determined by what is needed to mix withthe substantially pure oxygen from the ASU and provide the combustionoxidant with the desired O₂/CO₂ ratio. The dilute hydrocarbon stream 170can be mixed directly with the recycle CO₂ stream (e.g., with the streamin line 135 and/or line 165 and/or a side stream taken from line 135and/or line 165). The amount of the recycle CO₂ stream used in thismixture is sufficient to maintain the necessary mass flow through therecycle circuit and depends on the mass flow of the dilute hydrocarbonstream (this also provides a mechanism to handle changes in the flowrate of the dilute hydrocarbon streams). The remainder of the CO₂ fromthe turbine exhaust stream becomes export CO₂ fraction that will becleaned and sent to a pipeline for downstream utilization orsequestration.

The export CO₂ fraction and diluting CO₂ fraction streams may becompressed and pumped together in the typical operation of the powercycle (i.e., may be compressed and pumped in any manner of combinationsdepending on the final use of the export CO₂ fraction). In oneembodiment, these streams may be sent to a CO₂ purification unit (forexample, using refrigeration and distillation) to remove excess O₂ andany inert materials and generate a stream of high purity CO₂ at thedesired pressure. The diluting CO₂ fraction is then sent to be mixedwith incoming O₂ to form the high pressure oxidant needed in thecombustor. In another embodiment, the diluting CO₂ fraction can be sentdirectly to O₂ mixing without this impurity removal being required. Theexport CO₂ fraction is sent to a pipeline for downstream sequestrationor utilization.

In one embodiment, the recycle CO₂ stream can be mixed with the dilutehydrocarbon stream 170 prior to compression and pumping to the combustorinput pressure (e.g., about 300 bar in some embodiments). As illustratedin FIG. 2, a dilute hydrocarbons from hydrocarbon source 171 flowsthrough line 172 and is input to line 135. As such, the dilutehydrocarbon in line 171 is input to the line 135 comprising the recycleCO₂ working fluid downstream of the recuperator heat exchanger 120 andupstream of the compressor 140 and/or the compressor 160. This can bedone separately from the export CO₂ fraction and the diluting CO₂fraction to prevent contamination of these streams by hydrocarbons andother non-CO₂ species present in the dilute hydrocarbon stream 170. Thiscan be accomplished using either entirely separate rotating equipment orusing separate wheels of the same rotating equipment, as would befeasible in an integrally geared compressor. The mixed dilutehydrocarbon/recycle CO₂ stream (now at a pressure of about 300 bar andat a temperature slightly above ambient temperature) is then sent to theprimary heat exchanger train 120 to be heated against the turbineexhaust stream in line 112. Unless otherwise indicated, other elementsillustrated in FIG. 2 are as described in relation to FIG. 1.

As the stream is heated through the heat exchanger train 120 to atemperature near that of the turbine exhaust, hydrocarbons input via thedilute hydrocarbon stream 170 undergo thermal oxidation withoutsubstantial combustion. The thermal oxidation takes place withoutsubstantial combustion in that the conditions do not allow for formationof a sustained flame. Thus, the absence of substantial combustion doesnot necessarily exclude any combustion from occurring, and a smallpercentage (e.g., less than 5% by volume) of the hydrocarbon compoundsprovided via the dilute hydrocarbon stream may combust whilesubstantially all (e.g., at least 95% by volume) of the hydrocarboncompounds provided via the dilute hydrocarbon stream instead undergothermal oxidation. In some embodiments, thermal oxidation may take placein the complete absence of any combustion of the hydrocarbon compoundsprovided via the dilute hydrocarbon stream. This thermal oxidation mayoccur in the primary recuperator heat exchanger and/or may occur in aseparate heat exchanger that is dedicated to facilitating thesereactions. In some embodiments, thermal oxidation can occur withindedicated passages of the recuperator heat exchanger.

These oxidation reactions are enabled by the fact that the power cyclecombustor operates with an excess of O₂, leading to residual O₂ beingpresent in the recycle CO₂ stream at a substantially small concentrationbut at a high partial pressure. For example, the recycle CO₂ stream inline 135 and/or line 165 may have an O₂ concentration of about 0.01% byvolume to about 10% by volume, about 0.1% by volume to about 8% byvolume, or about 0.2% by volume to about 5% by volume. In the presenceof this O₂, entrained hydrocarbons (as well as other diluent species,such as H₂S) input to the recycle CO₂ stream from the dilute hydrocarbonstream begin to oxidize within the channels of the power cycle heatexchangers as they are progressively heated.

The mixture of the recycle CO₂ stream with the dilute hydrocarbon streamis preferably controlled such that the total hydrocarbon content of themixture is below the lower explosive limit (LEL), which can vary basedupon the specific mixture of compounds present. Thus, in someembodiments, the mixture of the dilute hydrocarbon stream and therecycle CO₂ stream can have a minimum hydrocarbon concentration of atleast 0.1% by volume, at least 0.5% by volume, at least 1% by volume, orat least 2% by volume, and the mixture of dilute hydrocarbon stream andthe recycle CO₂ stream can have a maximum hydrocarbon concentration thatis less than the LEL, as noted above. As a non-limiting example, amixture comprising predominately CO₂ and methane may have a maximummethane content of less than 5% by volume (e.g., about 0.01% by volumeto 4.95% by volume).

It is understood that conditions for combustion require the combinationof an ignition source with both of a fuel and an oxidant in a sufficientratio. When the fuel concentration is below the LEL, the fuel to oxidantratio is insufficient for combustion. Examples of LEL values for varioushydrocarbons are as follows (with all percentages being on a volumebasis): butane (1.8%); carbon monoxide (12.5%); ethane (3.0%); ethanol(3.3%); ethylene (2.7%); gasoline (1.2%); methane (5.0%); methanol(6.7%); and propane (2.1%). Based upon known LEL values, it is possibleto calculate the LEL of a substantially pure hydrocarbon fuel as well asa mixed hydrocarbon fuel to ensure that that the hydrocarbonconcentration is below the overall LEL for the particular material ormaterials being mixed with the recycle CO₂ stream. Since theconcentration of hydrocarbons in this mixed stream is so dilute (i.e.,being below the LEL of the mixture), “combustion” does not occur. Thisprocess simply oxidizes the hydrocarbons to CO₂ and water and producessensible heat for the recycle CO₂ stream, thereby allowing the highgrade heat of the turbine exhaust to be further preserved and useddownstream in the heat exchanger. This additional heat also reduces theneed for sources of low-grade heat used to optimize the power cyclerecuperative heat exchanger train. Namely, it may not be necessary toscavenge heat from the ASU main air compressor and/or the hot gascompression cycle as non-turbine derived heat sources.

The turbine exhaust in line 112 from this process is cooled in theprimary heat exchanger 120 as in a typical power production cycleconfiguration, such as shown in FIG. 1; however, it is then sent to amodified direct-contact cooler that has been upgraded to remove anySO_(x) and/or NO_(x) species arising from the dilute hydrocarbon stream(e.g., sulfate or sulfite species formed by oxidation of sulfurcontaining compounds, such as H₂S and/or nitrate or nitrite speciesformed by oxidation of nitrogen). An exemplary process in this regard isdescribed in U.S. Pat. application Ser. No. 15/298,975, filed Oct. 20,2016, the disclosure of which is incorporated herein by reference. Thecleaned turbine exhaust is then split into the diluting CO₂ fraction,the export CO₂ fraction, and the recycle CO₂ stream, and the processrepeats with additional dilute hydrocarbon stream being input to thepower cycle.

In some embodiments, the recycle CO₂ stream and the dilute hydrocarbonstream can be mixed within the primary heat exchanger train once therecycle CO₂ stream has been heated to an appropriate temperature tofacilitate the oxidation reactions. Alternatively (or in combination),the recycle CO₂ stream and the dilute hydrocarbon stream can be mixedwithin a further, separate heat exchanger. This can prevent thesereactions from occurring in the lower temperature portions of the heatexchanger train where the temperature may be insufficient to provide forthe oxidation reaction to occur. Accordingly, the recycle CO₂ stream maybe input to the heat exchanger at a first temperature section, and thedilute hydrocarbon stream may be input to the heat exchanger at asecond, higher temperature section wherein the temperature of therecycle CO₂ stream is sufficient to facilitate oxidation of thehydrocarbon compounds in the dilute hydrocarbon stream. As an example,in FIG. 3, an optional, second heat exchanger 167 (or supplemental heatexchanger) is illustrated. A side stream 166 taken from line 165 directsa portion of the recycle CO₂ stream through the second heat exchanger167 to be heated by oxidation of the dilute hydrocarbon stream in line172 that is input to the second heat exchanger 167 and is received fromhydrocarbon source 171. The heated stream of recycle CO₂ stream is theninput to the recuperator heat exchanger 120.

In some embodiments, the dilute hydrocarbon stream can be introducedinto the oxidant stream, which is formed of a mixture of oxygen and thediluting CO₂ fraction, at an appropriate location within the primaryheat exchanger (or alternatively a separate, dedicated heat exchanger)such that the temperature of the combined stream is sufficient tosustain the oxidation reactions. Using the oxidant stream can serve toincrease the rate (and decrease the required residence time) of thesereactions due to the higher partial pressure of oxygen present in suchstream relative to the partial pressure of oxygen in the recycle CO₂stream. For example, referring to FIG. 4, a diluting CO₂ fraction inline 165 a is taken from line 165 and mixed with oxidant in line 62 fromthe oxidant source 60 to form a diluted oxidant stream (e.g., with anO₂/CO₂ ratio of about 5/95 to about 40/60 or about 10/90 to about30/70). The diluted oxidant stream may be heated by passage through theheat exchanger 120 against the cooling turbine discharge stream in line112. All or a portion of the dilute hydrocarbon stream thus may be inputto the diluted oxidant stream prior to or during passage through theheat exchanger 120. As illustrated in FIG. 4, dilute hydrocarbon fromdilute hydrocarbon source 171 is passed through line 172 for input tothe diluted oxidant stream in line 62 downstream from the point whereCO₂ is added in line 165 a.

In some embodiments, a portion of the oxidant stream may be introducedinto the mixture of the dilute hydrocarbon stream and the recycle CO₂stream either upstream of or within the primary heat exchanger train (oralternatively a separate, dedicated heat exchanger). Such addition canserve to increase the partial pressure of oxygen and increase the rateof the oxidation reactions.

In some embodiments, a catalyst may be used in the area of the heatexchanger with oxidation is to occur in order to facilitate theoxidation reactions and ensure complete oxidation. As a non-limitingexample, commonly used water gas shift catalysts (e.g., various metaloxides, such as Fe₂O₃, Cr₂O₃, and CuO) may be used. Similarly, othercatalysts adapted to reduce the partial pressure of O₂ that is requiredin the mixed recycle CO₂ stream and dilute hydrocarbon stream may beused.

In addition, catalyzed oxidation can be carried out in a dedicatedreactor that is separate from the primary heat exchange unit 120. Asillustrated in FIG. 5, an optional oxidation reactor 180 can be used,and all or part of the dilute hydrocarbon stream can be input directlyto the oxidation reactor. In particular, dilute hydrocarbon from dilutehydrocarbon source 171 is passed through line 172 to oxidation reactor180 wherein the dilute hydrocarbon is oxidized to produce heat. Further,optionally, oxidant can be taken from line 62 (or directly from oxidantsource 60) in stream 62 a and can be input to the oxidation reactor 180.The oxidation of the dilute hydrocarbon stream in the oxidation reactor180 can produce a reaction stream 182 that can have a chemistrysubstantially comprised of CO₂ and H₂O (with a possible negligibleamount of residual hydrocarbons). The reaction stream 182 would beexpected to be increased in temperature as a result of the oxidationreaction, and the so-heated reaction stream can be input to the heatexchanger 120 at the appropriate temperature interface. In someembodiments, a portion of the recycle CO₂ stream (e.g., from one or bothof lines 135 and 165) may be added to the dilute hydrocarbon streamand/or the reaction stream 182. As seen in FIG. 5, CO₂ in line 165 b(taken from line 165) can be input to the line 172 with dilutehydrocarbon via line 165 b′ and/or to the reaction stream 182 via line165 b″. Such additions can be useful for regulating the temperature ofthe oxidation reaction in the oxidation reactor 180 and/or regulatingthe temperature of the reaction stream 182 itself before introduction tothe primary recuperative heat exchanger train 120. In this manner, thedilute hydrocarbon stream can essentially be used as a low grade heatsource that may be considered to be “external heat” for addition to therecuperative heat exchanger 120 that can add to or replace other sourcesof external heating, such as utilizing heat recuperation from the ASUand/or a hot gas recompression cycle. Such a manner of operation can beuseful to improve efficiency by reducing the UA requirements of thepower cycle recuperative heat exchanger train while providing additionalCO₂ for export and further offsetting fuel demand at the power cyclecombustor.

In some embodiments, the dilute hydrocarbon stream may be mixed with theturbine exhaust stream so that oxidation of the hydrocarbons can“super-heat” the turbine exhaust stream. As illustrated in FIG. 6,dilute hydrocarbon from dilute hydrocarbon source 171 can be inputthrough line 172 directly to the turbine exhaust in line 112 upstreamfrom the heat exchanger 120. This can serve to increase the amount ofheat available for recuperation by the recycle CO₂ stream upon passagethrough the heat exchanger 120. Oxidant from the oxidant stream 62 maybe input to the turbine exhaust stream in such embodiments dependingupon the residual oxygen concentration in the turbine exhaust and thechemistry of the dilute hydrocarbon stream 170. Such optional embodimentis illustrated in FIG. 6 wherein oxidant in line 62 b is passed fromline 62 (or directly from oxidant source 60) to the turbine exhaust line112. Although oxidant line 62 a is shown entering turbine exhaust line112 upstream of the point where the dilute hydrocarbon in line 172 isinput, it is understood that the oxidant line 62 a may enter the turbineexhaust line 112 downstream of the point where the dilute hydrocarbon inline 172 is input, or the oxidant line 62 a may connect directly to theline 172 for mixture with the dilute hydrocarbon prior to entry toturbine exhaust line 112.

In further, optional embodiments, as illustrated in FIG. 7, a portion ofthe turbine exhaust in line 112 can be diverted in line 112 a to anoxidation reactor 190 (which may include one or more catalysts as notedabove) to be combined with dilute hydrocarbon delivered in line 172 fromdilute hydrocarbon source 171. Further, optionally, oxidant from oxidantsource 60 may be input to the oxidation reactor 190. The line 112 a canbe configured to divert a portion of the expanded CO₂ working fluidupstream of the recuperator heat exchanger 120 and downstream from theturbine 110. The reaction product stream in line 192 exiting theoxidation reactor 190 will be elevated in temperature above thetemperature of the turbine exhaust in line 112 and can be furtherexpanded across a further turbine 195 (i.e., a secondary turbine or asupplemental turbine) for added power generation. The turbine exhaust instream 197 can be re-combined with the turbine exhaust in line 112 priorto entry to the recuperative heat exchanger 120, may be utilized forother purposes, or may be exhausted.

In any of the embodiments described herein, the dilute hydrocarbonstream may be supplemented with another fuel in order to accommodatechanges in the flow rate or composition of the dilute hydrocarbonstream. For example, a content of natural gas may be mixed with thedilute hydrocarbon stream.

The presently disclosed systems and methods are beneficial for theintegration of a high efficiency power production system with low BTUfuels without necessitating changes to the basic nature of the equipmentutilized (e.g., the combustor and/or turbine). The ability to utilizedilute hydrocarbon streams in this manner without the requirement forupgrading provides significant economic and process advantages, such asreducing or eliminating GPU requirements and/or increasing CO₂ recovery.

Many modifications and other embodiments of the invention will come tomind to one skilled in the art to which this invention pertains havingthe benefit of the teachings presented in the foregoing description.Therefore, it is to be understood that the invention is not to belimited to the specific embodiments disclosed and that modifications andother embodiments are intended to be included within the scope of theappended claims. Although specific terms are employed herein, they areused in a generic and descriptive sense only and not for purposes oflimitation.

1-26. (canceled)
 27. A method for heating a stream, the methodcomprising: compressing a stream comprising CO₂ to form a compressedstream comprising CO2 _(;) firstly, heating the compressed streamcomprising CO₂ in a recuperator heat exchanger with heat withdrawn froma stream of combustion products arising from combustion of a first fuelsource; and secondly, heating the compressed stream comprising CO₂ withheat obtained by oxidizing, without substantial combustion, a secondfuel source that is a dilute hydrocarbon stream, said oxidizing beingcarried out separate from the combustion of the first fuel source. 28.The method of claim 27, wherein the concentration of hydrocarbons in thedilute hydrocarbon stream is below the lower explosive limit (LEL) ofthe hydrocarbons.
 29. The method of claim 27, wherein the hydrocarbonsin the dilute hydrocarbon stream are catalytically oxidized.
 30. Themethod of claim 27, wherein the compressed stream comprising CO₂, afterbeing firstly and secondly heated, is passed through a combustor whereinthe first fuel is combusted with oxygen to form the stream of combustionproducts.
 31. The method of claim 30, wherein the stream of combustionproducts is expanded in a turbine to produce power before being passedto the recuperator heat exchanger.
 32. The method of claim 27, whereinthe dilute hydrocarbon stream is added to the compressed streamcomprising CO₂ before the compressed stream comprising CO₂ is input tothe recuperator heat exchanger.
 33. The method of claim 32, wherein thehydrocarbons in the dilute hydrocarbon stream are oxidized within therecuperator heat exchanger.
 34. The method of claim 32, wherein thehydrocarbons in the dilute hydrocarbon stream are oxidized in a furtherheat exchanger.
 35. The method of claim 27, wherein the dilutehydrocarbon stream is combined with the compressed stream comprising CO₂in the recuperator heat exchanger.
 36. The method of claim 27, whereinthe dilute hydrocarbon stream is input to an oxidation reactor.
 37. Themethod of claim 36, wherein a reaction stream exiting the oxidationreactor is input to the recuperator heat exchanger.
 38. The method ofclaim 36, wherein a reaction stream exiting the oxidation reactor isinput to a turbine for power production.